Yes, I see it as a very brutal (and probably effective) form of military demoralization, with a huge psychological component. Imagine the effects on morale of seeing a few thousand of your kinsmen impaled on giant pikes. Of course, as psychological warfare it is fairly crude. More advanced psychological warfare techniques aim to plant the seeds of doubt in one's own cause. In this case, it's not as sophisticated. It's a pure fear tactic, but if done in large enough numbers it also becomes a kind of implied concentration of force.

They didn't have media or quick means of communication back then. In the 20th century you could drop a 50 megaton bomb. In the 15th century, you put people on pikes. Let's also not forget that Romania was dealing with Islamists here, whose militaristic violence formed the entire grounds for the spread of their politico-religious system. If you are trying to send a message to dogs, you send it in a way that the dog understands. It makes no sense to hold some diplomatic standard of ethics when you're dealing with a program like Islam, the currency of which has only ever been uncompromising barbarism.

Crassus did the same thing on the Appian Way following the third servile war (the Spartacus one). Crucified 6,000 slaves as a warning to others. But Appian Way was one of the main routes into Rome and they were just left there to rot. Place must’ve stank for normal Romans!

Tens of thousands is the books are correct.

Yes, This is the history I have read.